Conventional hearing aids amplify all received sounds to increase their loudness. In most instances, however, the hearing loss of a deaf or partially deaf person is different at different frequencies. Furthermore, the range of intensities which the partially deaf person can comfortably hear is often different at different frequencies. More recent hearing aids, therefore, have been constructed so that their amplification is frequency and level dependent. In this way the amplified signal can be matched to the impaired hearing of the user of the aid.
One example of such a "matched" hearing aid is one that was developed in 1973. This well-known hearing aid uses a multi-band compression system, and is set up so that the signal from each frequency channel (band) varies over a reduced range of levels. This hearing aid can be used to fit the residual hearing of its user fairly accurately, with the result that a user certainly hears the incident sound in each frequency band equally.
Such a hearing aid, however, does not permit the user of the aid to discriminate easily between different sounds. It has now been found that this inability to discriminate between sounds is due to the reduction in the range of spectral shapes that are associated with different speech sounds.